Corrosion is an electrochemical process consisting of an anodic reaction, which involves metal ionization or oxidation (corrosion reaction), and a cathodic reaction based on the reduction of chemical species, causing deterioration of the metals physical and chemical properties, which in turn accelerates its aging and destruction.
Bio-corrosion or microbiologically influenced corrosion (MIC) occurs mostly in stagnant conditions or in operations with low or intermittent fluid flow, and represents a serious problem that affects various industries. There are no official figures on the cost caused by MIC, but an indication of its importance can be obtained from individual companies or industrial sectors.
MIC is an electrochemical process in which microorganisms cause the deterioration of a material, usually a metal, either directly or indirectly, due to their production of extracellular polymeric substances (EPS), organic and inorganic acids and volatile compounds such as ammonia or hydrogen sulfide.
Microorganisms promote electrochemical oxidation reactions and reduction of sulfates, sulfites, nitrates and sulfur in the presence of an electrolyte where there is oxygen consumption by microbial communities, causing cathodic depolarization of the metal.
The corrosion type and rate caused by microorganisms is dependent directly on the availability of adequate nutrients in the environment.
Pitting corrosion on metal surfaces is the corrosion type most associated with microbial activities.
There are various types of microorganisms involved in the processes of metal bio-corrosion. The bacteria commonly associated with these processes are sulphate reducing bacteria (SRB), metal reducing bacteria, sulfide oxidizers, secretors of organic acids and extracellular polysaccharides (EPS).
Biofilms
A biofilm is a microbial mass made of bacteria, fungi, algae and other microorganisms, which usually forms in four stages:    1. Conditioning—the surface for adhesion of pioneer microorganisms is conditioned;    2. Adhesion—the adhesion of pioneer bacterial species and their reproduction takes place, and colonization begins;    3. Colonization—the colony of microorganisms is created and extracellular polymeric substances (EPS) are produced, favoring the formation and growth of the biofilm;    4. Accumulation—the biofilm is fully developed, forming a differential aeration zone between the biofilm and the metal surface through mechanisms of polymer-metal interactions.
The extracellular polymeric substances (EPS) are polysaccharides derived from Gram-negative and Gram-positive cells. They promote the initial adhesion of microorganisms to solid surfaces, the formation and maintenance of biofilms, resistance to environmental factors and allow microorganisms to capture nutrients.
A biofilm is a microbial cell community structure, which produces matrix exopolymers. They inhabit regions between oxic and anoxic layers, and these adhere to both inert or living material. (Costerton et al, “Bacterial Biofilms: A Common Cause of Persistent Infections”, Science (www.sciencemag.org), 21 May 1999, Vol. 284, 284. 1318-1322).
(The concept of a biofilm, which involves the term microbial communities, comes from B. Carpentier and O. Cerf in “Biofilms and Their Consequences, particularly with reference to hygiene in the food industry,” Journal of Applied Bacteriology 1993, 75, 499-511: “a community of microorganisms embedded in an organic polymer matrix, adhering to the surface”).
For bacteria to survive and reproduce successfully in many systems, they require the colonization of a surface and/or integration into a community that has formed a biofilm.
In an aqueous system, a microorganism is subject to various forces such as gravity and fluid drag force, which is proportional to the speed with which it moves. Gravity facilitates their transport and bond with the surface.
Once the microorganism has been transported to the substrate surface, initial binding takes place. This is described as a two phase event:
1) Reversible binding phase
2) Irreversible binding phase.
Adhesion is the principle stage, in which a bacterium performs surface colonization. This colonization increases with increasing surface roughness, since there is a greater surface area and the separation forces decrease.
It is important to note that a key element to achieve biofilm formation is that the flow in which it is growing is in the laminar regime, so that it does not detach from the surface to which it is adhered.
Hydrodynamics also plays an important role in the biofilm development, as these organizations develop on a liquid-solid interface where the flow speed that passes across it influences the microorganism's physical detachment. The biofilms also possess a canal system in which water flows, allowing them to transport nutrients, oxygen and waste.
In order to study adhesion of microorganism such as bacteria and their biofilm formation under flow conditions, laboratory model systems have been used, the design of which results in bioreactors working under the desired conditions.
The maximum number of attached bacteria per square centimeter is a parameter that allows one to characterize and determine the biofilm formation dynamics, which can be much slower and proportionally lower in a discontinuous and turbulent system than in a laminar flow system. The biofilm formation rate and thickness are not as dependent on the carbohydrates availability (glucose or lactate) or degree of consumption of any substrate, as they are on iron salts. The number of bacteria on the surface is influenced by the presence of other bacterial species, which reduces the number of cells in the biofilm. The number of bacteria on the surface is quantified in order to evaluate the influence of environmental factors on adhesion and biofilm formation using a combination of fluorescence, atomic force and environmental scanning microscopy, ultrasonic surface bacteria remover and indirect conductimetry.